this is a scam, you never get the money!
So don't sell your gear to someone with a chque or this story:

We are interested in buying your above mentioned products at advertised rate. Goods to be picked up from your location for onward shipping to London. Cost and modalities of shipping to be born by us. Payment for the purchase is by check through a debtor in US. The debtor owes us higher than our purchase. He will therefore be sending you a check for $5,000 to cover the cost of our order, as well as the Shipping costs. On receipt of the check, you will cash and deduct the cost of our order .The balance you will send as we would instruct you to the Agency that will ship the Goods to us. If this arrangement suites you fine, please send us your address where to send the check, as well as the name to be on it. Also send us your phone number for easy communication. Looking forward to hearing from you. Thank you. Best Regards MR. JOHN MARLES.